According to a configuration of the internal combustion engine injection system, a fuel unit pump is provided in order to supply fuel under pressure to the fuel injectors (injector nozzle). The fuel unit pump is actuated by a corresponding cam lobe of a rotating shaft of the internal combustion engine, for example the camshaft or crankshaft. More in detail, the fuel unit pump is provided with a roller tappet that is contacted by the camshaft, in a cam—cam follower configuration. In particular, the cam lobe of the camshaft acts as the cam and the roller tappet acts as the cam follower. The roller tappet is connected to a reciprocating element of the fuel unit pump, so that the rotary movement of the camshaft can be transmitted to the fuel unit pump, and in particular to a reciprocating element of the fuel unit pump actuated by the contact of the roller tappet with the cam lobe of the camshaft.
In fact, the roller tappet is provided with a cam roller having a rotation axis arranged perpendicularly to the longitudinal movement direction of the above mentioned reciprocating element. The cam roller is contacted by the cam lobe(s) of the camshaft, so that the rotary movement of the camshaft can be transformed in a linear movement of the roller tappet and thus of the reciprocating element of the fuel unit pump, connected thereto. The fuel unit pump is fluidly connected to the fuel injectors, preferably by means of a fuel rail, to supply fuel in the engine cylinder.
However, a very high precision is required to assure that, when the fuel unit pump is mounted in the internal combustion engine (preferably in the cylinder head or in the engine block of the internal combustion engine), the roller tappet, and in particular the cam roller of the roller tappet, is correctly aligned with respect to the camshaft. In other words, it should be assured that the axis of rotation of the cam roller is exactly parallel to the rotation axis of the rotatable shaft, e.g. the rotation axis of the camshaft. As a result, the lateral surface of the cam roller can properly contact the lateral surface of the relevant cam lobe of the camshaft.
However, due to certain circumstances, e.g. machining errors and tolerances, it is difficult to satisfy the above mentioned conditions, so that misalignments can occur between the cam lobe and the cam roller. In order to avoid a configurations where the contact between the two elements is only punctual (the so called “edge effect”), the cam roller has a so called “crowning” or “logarithmic” profile. These profiles avoid punctual contact but, on the other side, limit the maximum possible contact area between the two elements. As a result, higher stresses are generated on the cam roller, and the cam roller is generally dimensioned larger than required. As a result, cam rollers can be complex and costly.
Moreover, the size of the cam roller cannot be increased at pleasure, so that a limit is imposed also to the fuel pressure handled by the fuel unit pump. Furthermore, because of the problem caused by misalignments, machining tolerance of the fuel unit pump and of the portions of the internal combustion engine cooperating with the fuel unit pump should be very strict.